The scale on a map is 3cm : 4km. If the distance between two cities is 20km, how far apart in cm are the two cities on the map?
Explanation: The scale means that every 3cm on the map corresponds to an actual distance of 4km. An actual distance of 20km is the same as 5 $\cdot$ 4km. The distance between the two cities on the map is 5 $\cdot$ 3cm, or 15cm.